I wrote:
> However ... I do not find any indication in the GNU tar docs
> that it produces sparse files by default. It looks like you
> need to say -S/--sparse to make that happen. Maybe you have
> a version that's been hacked to make that the default?
Bleah. Digging in the man pages at freebsd.org, I read
--read-sparse
(c, r, u modes only) Read sparse file information from disk.
This is the reverse of --no-read-sparse and the default behav-
ior.
It's apparently been there and been default since FreeBSD 13.1.
This leads one to wonder how come BF member dikkop is managing
to run this test successfully. I speculate that it's using a
filesystem type that doesn't do sparse files (cc'ing Vondra
for confirmation on that).
It looks like to make this test stable on modern FreeBSD,
we need to see if tar accepts --no-read-sparse and use that
switch if so.
regards, tom lane
